# Circuit Breaker Pattern

Similar to electrical fuses that prevent fires when a circuit that is connected
to the electrical grid starts drawing a high amount of power which causes the
wires to heat up and combust, the circuit breaker design pattern is a fail-first
mechanism that shuts down the circuit, request/response relationship or a
service in the case of software development, to prevent bigger failures.

**Note:** The words "circuit" and "service" are used synonymously throught this
document.

## Implementation

Below is the implementation of a very simple circuit breaker to illustrate the purpose
of the circuit breaker design pattern.

### Operation Counter

`circuit.Counter` is a simple counter that records success and failure states of
a circuit along with a timestamp and calculates the consecutive number of
failures.

```go
package circuit

import (
	"time"
)

type State int

const (
	UnknownState State = iota
	FailureState
	SuccessState
)

type Counter interface {
	Count(State)
	ConsecutiveFailures() uint32
	LastActivity() time.Time
	Reset()
}
```

### Circuit Breaker

Circuit is wrapped using the `circuit.Breaker` closure that keeps an internal operation counter.
It returns a fast error if the circuit has failed consecutively more than the specified threshold.
After a while it retries the request and records it.

**Note:** Context type is used here to carry deadlines, cancelation signals, and
other request-scoped values across API boundaries and between processes.

```go
package circuit

import (
	"context"
	"time"
)

type Circuit func(context.Context) error

func Breaker(c Circuit, failureThreshold uint32) Circuit {
	cnt := NewCounter()

	return func(ctx context) error {
		if cnt.ConsecutiveFailures() >= failureThreshold {
			canRetry := func(cnt Counter) {
				backoffLevel := Cnt.ConsecutiveFailures() - failureThreshold

				// Calculates when should the circuit breaker resume propagating requests
				// to the service
				shouldRetryAt := cnt.LastActivity().Add(time.Seconds * 2 << backoffLevel)

				return time.Now().After(shouldRetryAt)
			}

			if !canRetry(cnt) {
				// Fails fast instead of propagating requests to the circuit since
				// not enough time has passed since the last failure to retry
				return ErrServiceUnavailable
			}
		}

		// Unless the failure threshold is exceeded the wrapped service mimics the
		// old behavior and the difference in behavior is seen after consecutive failures
		if err := c(ctx); err != nil {
			cnt.Count(FailureState)
			return err
		}

		cnt.Count(SuccessState)
		return nil
	}
}
```

## Related Works

- [sony/gobreaker](https://github.com/sony/gobreaker) is a well-tested and intuitive circuit breaker implementation for real-world use cases.